Fill orbitals by increasing energy using the Aufbau principle.
Electron configuration describes how electrons are distributed among atomic orbitals. The Aufbau (building-up) principle fills orbitals in order of increasing energy: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s... Each orbital type has a maximum capacity (s=2, p=6, d=10, f=14). Hund's rule requires electrons to fill degenerate orbitals singly before pairing, and the Pauli exclusion principle limits each orbital to two electrons with opposite spins. Notable exceptions include Cr ([Ar] 3d⁵4s¹) and Cu ([Ar] 3d¹⁰4s¹), where half-filled or fully filled d-subshells provide extra stability. Electron configuration determines an element's chemical properties, position in the periodic table, magnetic behavior, and spectroscopic characteristics.